Six hours
can kill a reef tank - especially one with a heavily populated DSB, or one that is, um, "close to the edge" stocking-wise (and who isn't, at one point or another?).
Here's how I've addressed this problem - both of my home tanks are set up with the main circulation pump connected to an UPS (computer battery back up). A $100-$130 APC (American Power Company) Back-UPS unit will run a pump or two for 3-4 hours. Into this UPS I also have plugged in some PennPlax B-11 battery powered airpumps (these pumps are plug in, but only kick on when the power to the plug stops, supplying air to an airstone, which would at least provide some water movement and aeration).
The other nice thing about these units is that they have an audible alarm that rings when the power goes off (and yes, you can shut the alarm off after it rings), so you know if the power is off (even if you're sleeping < VBG>
.
I did this because I live in California; land of the earthquake, so longer power outages can be expected, from time to time. the way it works (I've tested it) is the power goes off and the UPS takes over, keeping my main circulation pumps running for 3-4 hours. When the UPS dies, the PennPlax pumps kick on, keeping the water moving (I have separate PennPlax pumps in the tank and refugiums). These pumps will run on a "D" battery (or two, I forget) for up to 48 hours. That's not generally necessary in my case, as I have a gas generator that I can use during daylight (really too noisy to use at night). The generator runs during daylight, powering everything, and recharging the UPS, the UPS and PennPlax run at night when I shut off the generator, keeping the water oxygenated.
Water temp really isn't much of an issue in SoCal, in your case, you could get a more expensive UPS so that it can run the heater a while longer Or get a separate UPS for the heater).
Just get the wattage/amperage draw on your main circulation pump (s), then go to the local Best Buy/computer store and look at the battery back up systems. They will have a chart on the box that tells you how long each unit will run under a given draw.
$100-$200 for the UPS and PennPlax (like $7-$10 each) combined really isn't that much to pay for insurance when you start thinking about how much money you have invested in your tank.
A generator is considerably more expensive ($500-$1000), but I convinced my significant other that we could use it to run the refrigerator and freezers in the event of a prolonged outage, saving money on food spoilage (but it's the tanks that get hooked up first, lol) - not to mention running the Nintendo for the kids......
Sorry this was so long, but I thought it important to post what I've done and why I did it that way.
Oh yeah, part of the "why," I like everything to be as automatic as possible. The first time I had a major outage (8+ hours), My family was 400 miles away from home (seems a tree fell in Oregon, taking out most of the California powergrid). I had to call the neighbor we had watching the house/feeding the animals, explain where the battery-operated pumps were, explain how and where to place them, etc. I decided then that there had to be an easier/more automatic way to ensure my tanks continued survival when I wasn't standing next to it (or still in the state and realizing a major power outage was occurring).
Kevin